1. Field of the Invention
The present invention relates to a gold plating solution and a gold plating method, particularly to a noncyanide type electrolytic plating solution and a gold electroplating method employing such a gold plating solution.
2. Discussion of Background
As a gold plating solution, a cyanide type plating solution has been known from long ago. By using a cyanide type gold plating solution, it is possible to deposit a gold plated film having excellent characteristics such that it is dense and has a smooth surface. Besides, the cyanide type plating solution is stable, and its control is easy. Accordingly, it is widely used. However, cyanide has a strong toxicity and has had many problems with respect to the working environment, the waste liquid treatment, etc.
Therefore, various noncyanide type low toxicity gold plating solutions have been proposed. For example, a gold plating solution has been widely used wherein gold sulfite is dissolved (JP-A-11-61480). However, with this gold plating solution, sulfite ions in the solution tend to be readily oxidized by dissolved oxygen or oxygen in the atmosphere, and the useful life as a gold plating solution tends to be short. Accordingly, it has been required to take a measure to prevent oxidation, e.g. by nitrogen sealing, during the storage or even during the plating operation, and thus, there has been a problem that its handling is cumbersome.
Further, a gold plating solution is also proposed in 10 which a thiosulfate/gold complex, a sulfite, boric acid and ethylene glycol are dissolved (JP-A-51-47539). However, also this gold plating solution has had the same problems as the above-mentioned gold plating solution employing gold sulfite, since sulfite ions in the plating solution tend to be oxidized.
Further, a gold plating solution obtained by dissolving a gold compound selected from the group consisting of various gold complexes such as acetylcysteine/gold complexes, and acetylcysteine as a complexing agent, or a gold plating solution containing at least one alkane sulfonic acid or alkanol sulfonic acid, gold ions and a non-ionic surfactant, has, for example, been proposed (JP-A-10-317183, JP-A-8-41676). However, like the gold plating solution containing gold sulfite, each of them contains monovalent gold ions, whereby deposition of gold by the reaction 3Au+→2Au+Au3+ is likely to take place, and thus, there is a problem of instability of the gold plating solution.
Therefore, a gold plating solution has been proposed wherein an ethylene diamine/gold complex as trivalent gold ions is dissolved (JP-A-11-293487, JP-A-2000-204496, JP-A-2000-355792, JP-A-2001-110832). However, ethylene diamine has a problem of harmful effects such that there have been cases of accidental death due to percutaneous and inhalation exposure (Chemical Substance Toxicity Handbook vol. II, II-84, (1999) published by Maruzen). Accordingly, a gold plating solution is desired which is safe and excellent in handling efficiency.
Further, after the applicants' Japanese Patent Application No. 2002-068691 filed on Mar. 13, 2002 from which the priority is claimed in the present application, a case has been reported wherein a gold plated film of black color was obtained by carrying out gold plating in the presence of an organic solvent when gold electroplating was carried out by means of a gold solution having gold dissolved in a solution containing iodine (I2) and iodide ions (I−) (2002 National Science Education General Assembly, vol. 24, p. 66-67). However, in this case, the crystal particles of gold in the gold plated film formed were coarse, whereby the gold plated film exhibited a black color, and a glossy beautiful gold plated film was not obtained.
As mentioned above, the prior art has had problems such as 1) a problem in the working environment or waste liquid treatment due to a toxic substance, 2) low chemical stability such as susceptibility to oxidation, and 3) thickening growth of gold crystal particles in the gold plated film. Especially when the gold crystal particles in a gold plated film are coarse, glossiness or smoothness of the gold plated film tends to be low, and it tends to be difficult to apply such a film to ornamental or electronic components. Accordingly, a gold plating solution is desired which is safe, chemically stable and excellent in handling efficiency and which is, at the same time, capable of forming a smooth gold plated film wherein gold crystal particles are fine and dense.